A bath or sink, herein “vessel”, is drained by a “waste-overflow” bath drain assembly. The upper portion of this assembly consists of an “elbow ”, sometimes called a “head fitting”, which is fastened to the vessel wall by a “tieplate” fastened by a variety of means through the overflow hole in the vessel wall into the elbow on the outer vessel wall. Common tieplate shapes include a simple tiebar across the width of the overflow hole, and round plates with bolt holes.
In the early days of plumbed baths, a perforated metal plate was fastened to this tieplate by means of a bolt which might also bear a chain suspending a plug. This fastened plate was called a “faceplate”. This terminology stuck even when the faceplate was deepened into a cup, the perforations moving off the vertical face plane to the bottom of the cup rim, out of aesthetic sight. Ball (U.S. Pat. No. 5,890,241: Apr. 6, 1999) is an example of a bolt-on faceplate.
Other means of securing the faceplate to the tieplate include snap-on faceplates (Dunnett U.S. Pat. No. 7,007,319: Mar. 7, 2006), hinged faceplate (Freville U.S. Pat. No. 4,796,310: Jan. 10, 1989), and faceplates secured with a set screw (Ball U.S. Pat. No. 6,173,459: Jan. 16, 2001). Often the attachment is integral to some more complex drain functionality.
While not related to waste-overflow bath drain assemblies, a mating slot and tab can be found in McIlvenna (U.S. Pat. No. 6,669,492: Dec. 30, 2003) and in In (U.S. Pat. No. 7,094,969: Aug. 22, 2006).
Covering the overflow hole with a faceplate performs mainly an aesthetic function. Indeed, an open overflow without a faceplate would provide the best functionality for the drainage functions of the overflow hole; namely,                (1) The overflow hole serves as a pressure relief vent when the water drains out of the waste drain in the bottom of the vessel. This avoids having the water being siphoned out of the water trap that is plumbed between the vessel drain and the pipe to the primary sanitary drain. Maintenance of water in this water trap is essential for preventing sewer gases from travelling upward into the vessel.        (2) The overflow hole mitigates overflowing of liquid over the brim of the vessel, hence its name.        (3) The overflow hole provides access to the household drain system as it permits the passing of a “snake” by the plumber to clear blockages. In general, contemporary tieplates and tiebars that fasten the rear elbow to the bath have (or consist of) a cross bar to which the faceplate is fastened. This prevents plumber snake access. Temporarily unscrewing the faceplate is not an option as the elbow would fall off behind the vessel.        
Nonetheless, in most vessel installations the user of the vessel has come to expect that the overflow hole be discretely covered by a faceplate, though even frontal perforations became unacceptable, leading to the evolution of the cup shaped faceplate with perforations on the rim underneath.